feat(io): save and reload display perimeters

A display perimeter written without its own tag reloaded as an ordinary
cut, so a saved blank would come back with its outline to be cut. Programs
now carry it as :DISPLAYPERIMETER and the reader maps it back, and stored
source entities keep the DISPLAY PERIMETER layer so editing the drawing
rebuilds the same never-cut outline.

Builds older than this read the unknown tag as a cut. Open nests with
display perimeters only in builds that include this change.

OpenNest.IO.Tests/DisplayPerimeterPersistenceTests.cs

@@ -0,0 +1,55 @@
+using OpenNest.Bending;
+using OpenNest.CNC;
+using OpenNest.Geometry;
+using OpenNest.TabbedCutouts;
+
+namespace OpenNest.IO.Tests;
+
+/// <summary>A supplied blank's display perimeter must survive save/reload as never-cut geometry.</summary>
+public class DisplayPerimeterPersistenceTests
+{
+    private static Drawing Door()
+    {
+        var outline = new List<Entity>
+        {
+            new Line(0, 0, 30, 0), new Line(30, 0, 30, 20), new Line(30, 20, 0, 20), new Line(0, 20, 0, 0),
+        };
+        foreach (var line in outline)
+            line.Layer = SpecialLayers.DisplayPerimeter;
+        var entities = new List<Entity>(outline)
+        {
+            new Line(5, 5, 10, 5), new Line(10, 5, 10, 10), new Line(10, 10, 5, 10), new Line(5, 10, 5, 5),
+        };
+        return CadImporter.BuildDrawing(new CadImportResult { Entities = entities, Name = "door" },
+            entities, new List<Bend>(), 1, null, null, null, new List<TabbedCutout>());
+    }
+
+    private static int DisplayPerimeterMoves(Program program) =>
+        program.Codes.Count(code => code is LinearMove { Layer: LayerType.DisplayPerimeter } or ArcMove { Layer: LayerType.DisplayPerimeter });
+
+    [Fact]
+    public void DisplayPerimeter_SurvivesSaveAndReloadOnTheDrawingAndItsPlacements()
+    {
+        var drawing = Door();
+        Assert.Equal(4, DisplayPerimeterMoves(drawing.Program));
+        var nest = new Nest("door") { Units = Units.Inches };
+        nest.Drawings.Add(drawing);
+        var plate = new Plate(new Size(20, 30));
+        plate.Parts.Add(new Part(drawing, Vector.Zero));
+        nest.Plates.Add(plate);
+        using var stream = new MemoryStream();
+        Assert.True(new NestWriter(nest).Write(stream));
+
+        var reader = new NestReader(new MemoryStream(stream.ToArray()));
+        var restored = reader.Read();
+
+        Assert.Empty(reader.Warnings);
+        var reread = Assert.Single(restored.Drawings);
+        Assert.Equal(4, DisplayPerimeterMoves(reread.Program));
+        var outline = CadImporter.EditableEntities(reread).Where(e => e.Length > 19.9).ToList();
+        Assert.Equal(4, outline.Count);
+        Assert.All(outline, e => Assert.Same(SpecialLayers.DisplayPerimeter, e.Layer));
+        var part = Assert.Single(restored.Plates.SelectMany(p => p.Parts));
+        Assert.Equal(4, DisplayPerimeterMoves(part.Program));
+    }
+}

OpenNest.IO/EntitySerializer.cs

@@ -191,6 +191,8 @@ namespace OpenNest.IO
                 return SpecialLayers.Break;
             if (string.Equals(name, SpecialLayers.Tab.Name, StringComparison.OrdinalIgnoreCase))
                 return SpecialLayers.Tab;
+            if (string.Equals(name, SpecialLayers.DisplayPerimeter.Name, StringComparison.OrdinalIgnoreCase))
+                return SpecialLayers.DisplayPerimeter;
 
             return new Layer(name);
         }

OpenNest.IO/NestWriter.cs

@@ -720,6 +720,9 @@ namespace OpenNest.IO
                 case LayerType.Tab:
                     return ":TAB";
 
+                case LayerType.DisplayPerimeter:
+                    return ":DISPLAYPERIMETER";
+
                 default:
                     return string.Empty;
             }

OpenNest.IO/ProgramReader.cs

@@ -288,6 +288,10 @@ namespace OpenNest.IO
                                         layer = LayerType.Tab;
                                         break;
 
+                                    case "DISPLAYPERIMETER":
+                                        layer = LayerType.DisplayPerimeter;
+                                        break;
+
                                     case "SUPPRESSED":
                                         suppressed = true;
                                         break;
@@ -394,6 +398,10 @@ namespace OpenNest.IO
                                         layer = LayerType.Tab;
                                         break;
 
+                                    case "DISPLAYPERIMETER":
+                                        layer = LayerType.DisplayPerimeter;
+                                        break;
+
                                     case "SUPPRESSED":
                                         suppressed = true;
                                         break;